Portable, hand-held data entry devices or "electronic notebooks", as they are sometimes called, have enjoyed increasing popularity in a wide variety of data gathering applications. Where, in the past, data was manually gathered and converted into computer compatible form for transmission to a remote main CPU, these portable data entry devices automatically convert the data into a suitable form for transmission to the CPU. The data entry devices may include onboard modems or separate interface formatter units for converting the digital data stored in memory to a form compatible for transmission over the telephone lines to the remote CPU. U.S. Pat. Nos. 3,976,840 to Cleveland et al; 4,068,095 to Ghormley et al; and 4,268,721 to Nielson et al are examples of the former and U.S. Pat. Nos. 3,942,157 and 4,016,542 both to Azure are representative examples of the latter.
Some of the applications for these devices include reordering of goods in grocery stores; monitoring station readings for pollution controls; meter reading in gas and utilities; generating route sales orders; field service applications; and data collection in scientific research.
While these prior art devices have certainly decreased the task of data gathering, they are somewhat lacking in versatility. For the most part the prior art data entry devices are designed exclusively for communication with a remote CPU via a modem over the telephone lines. They are not capable of communicating directly with a plurality of external hardware peripheral devices without requiring substantial modification of the data entry device design. Unfortunately, this lack of versatility wastes the potential resources available from the on-board microprocessor in those units employing one. The units such as those shown in the '721 patent do, of course, utilize internal self-contained miniaturized electronic equipment such as a liquid crystal displays, keyboards, and printers which may loosely be termed peripheral devices. However, these are to be distinguished from which shall be defined as "external peripheral devices" such as high speed printers, floppy disk memories, and the like which can be detachably removed to expand or contract the overall system configuration in a truly modular approach.
Several desk top computer terminals now available on the market do have this advantageous expansion capability. Unfortunately, while they are loosely referred to as portable terminals they are in reality bulky and cumbersome devices that do not lend themselves to use in those applications noted above where hand-held devices are commonly used. The desk top terminals generally employ two separate bus structures; an internal bus providing communication between the terminal microprocessor and on-board memory; and a system bus structure to which the external peripheral devices are connected. This dual bus structure requires the use of bus interfaces to provide the necessary timing signals and data formatting to enable communication between the external peripheral device and the terminal microprocessor. Generally there are two such bus interfaces; one between the hardware of the peripheral device and the system bus, and another interface between the system bus and the internal bus connected to the microprocessor. This approach requires the use of costly and space consuming hardware logic to provide the relatively sophisticated control signals to effectuate communication between the external peripheral device and the terminal microprocessor.